1. Field of the Invention
The present invention relates to a power converting device, and in particular to a DC to DC power converter which can improve light-load efficiency.
2. Description of Related Art
In electronic engineering, a DC-to-DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage lever to another, and the converted DC voltage is stabilized at a preset voltage value. Generally speaking, the DC-to-DC converter is divided into two categories: one is “step-down” DC-to-DC converter (namely, the output voltage is lower than the input voltage), and other one is “step-up” DC-to-DC converter (namely, the output voltage is lower than the input voltage). The DC-to-DC converter is mainly applied to a distributed power system. Hence, the DC voltage of the previous stage is fixed, and the DC voltage level of the next stage can be connected to the corresponding DC-to-DC converter according to the required power.
More particularly, the DC-to-DC converter can be separated into two categories: the pulse width modulation (PWM) converter and the resonant converter. The hard-switching operation of the PWM converter introduces the high switching losses and the poor efficiency. Accordingly, the soft-switching technology has been developed for the resonant converter to reduce the switching losses and increase the efficiency.
The DC characteristic of the resonant converter could be divided into ZVS (zero-voltage switching) region and ZCS (zero-current switching) region. Accordingly, the resonant circuit structure is adopted in high-efficiency and high-power power circuits.
Reference is made to FIG. 1, which is a circuit diagram of a conventional power converting device. The power converting device includes two switching elements Q1 and Q2, a first capacitor C1, a second capacitor C2, a first inductor L1, a second inductor L2, a first diode D1, a second diode D2, a transistor T1, and a controller 50. The switching elements Q1 and Q2 collectively construct a switching circuit to convert a direct current (DC) power source VIN into a pulsating signal. The pulsating signal then is transmitted to a resonant tank constructed by the first capacitor C1, the first inductor L1 and the second inductor L2 for generating resonant. After that, the signal is transmitted to a secondary side of the transistor T1, and then is converted into another DC power source with a voltage magnitude different from the DC power source VIN by the first diode D1, the second diode D2, and the second capacitor C2.
Reference is made to FIG. 2, which graphically represents inductances of second inductor versus current for the power converting device of FIG. 1. As shown in FIG. 2, in an unsaturated condition, the inductances of second inductor L2 are substantially fixed, thus the switching frequency of the switching elements Q1, Q2 is increased for stabilizing the output voltage when the power converting device is operated under light-load condition. However, the light-load efficiency of the power converting device is growing worse when the switching frequency is increased, as shown in FIG. 3. In FIG. 3, dotted line shows the frequency response of the power converting device under heavy-load condition, and solid line shows the frequency response of the power converting device under light-load condition.